Mrs. Krabs was or is the mother of Pearl Krabs and the wife of Eugene H. Krabs. It is unknown if she is deceased or if she is divorced from Eugene. In a book, Mr. Krabs states that Pearl takes after her mother, suggesting that Mrs. Krabs is a whale. It is possible that Mrs. Krabs is Pearl's adoptive mother, but does not have custody over her.
It is highly unlikely that she will ever make a physical appearance in the show. Pearl's lack of a mother is a reference to the phrase "mother of Pearl," which Mr. Krabs often says when something surprises him. Stephen Hillenburg is against revealing the identity of Mrs. Krabs and has kept it a secret throughout the series.
The exclamation "mother of Pearl!" is used in the following episodes, usually by Mr. Krabs: